This invention relates to a seismic sensor and more particularly to apparatus for responding to vibrations or impulse forces.
The term seismic is generally associated with earthquakes or attendant shaking of various structures which occurs during such an earthquake. Essentially a sensor device which has the capability of responding to a shock or vibration can generally be referred to as a seismic sensor. Such devices have found widespread use in regard to intrusion type systems. Such systems serve to monitor a premise to guard against unauthorized entry. It is, of course, well known that a burglar or other person could attempt to break a glass pane or a door or other structure to gain unauthorized access to a monitored area. In this manner, such sensors have been employed to respond to shock or vibratory forces which would normally accompany such a break-in.
In any event, the prior art is replete with a number of devices which are employed as shock and vibration sensors. Certain of these devices are generically referred to as accelerometers and will respond to an acceleration or force in a given plane. Accelerometer devices or vibration detectors, based on acceleration, are extremely expensive as they require various transducers such as those employing piezoresistive devices, variable reluctance, magnetostrictive as well as piezoelectric devices. Hence, the prior art, especially in the field of intrusion detection, sought to replace these devices with a simpler structure.
Thus, an extremely early arrangement of a mechanical vibration sensor is depicted in German Pat. No. 262,949. This patent employs a conductive sphere which is mounted upon conductive contacts arranged as a tripod. In this manner, vibration would cause the sphere to move away from the contacts, thus indicating an alarm. This feature is also disclosed in U.S. Pat. No. 3,560,680 issued on Feb. 2, 1971. Both structures are normally associated with unreliable operation and exhibit many false alarms when employed in conjunction with an intrusion system.
Other prior art devices employ a toroidal mass which also serves to sense shock or vibration in a particular manner and in a particular orientation. These devices are not acceptable for use in conjunction with intrusion systems. A particularly interesting device is depicted in U.S. Pat. No. 4,025,744 entitled SHOCK AND VIBRATION SENSITIVE SWITCH issued on May 24, 1977 to L. J. Kniskern. This patent purports to circumvent many of the problems of prior art and contains a number of additional references in the Background of Invention. The patent describes a shock and vibration sensor which includes a toroidal shaped disk having an outer conductive periphery. The disk is supported by two sharp penetrating contacts. The contacts engage the outer peripheral surface of the disk and when the sensor unit is subjected to a force or vibration, the toroidal disk moves, thus breaking contact and indicating an alarm condition.
There presently exists many detection circuits which will monitor the nature of the openings to determine whether a valid intrusion has occurred and systems for doing so are well known in the art. Accordingly, the sensor depicted in the above noted patent can be employed in an intrusion system to monitor unauthorized entries. An extreme problem associated with this transducer is that it has a limited capability in regard to mounting the same on a surface to be protected. The sensor depicted in this patent can only be mounted in two orientations; both of which are vertical and differ from each other by 180.degree.. In order to accomplish this, a particular set of contacts is necessary. The contact arrangement is depicted in the patent and incorporates two large C-shaped contact members which can engage the outer periphery of the disk dependent upon the disk orientation. Apart from this problem is the general problem of sensitivity of the structure as well as requiring a large conductive surface which is analogous to the entire outer surface of the toroid.
It is therefore an object of the present invention to provide an improved sensor which is extremely compact in configuration and which can be mounted in four distinct rotational positions to thereby give a user greater flexibility in mounting the device. The sensor to be described is more economical in that it does not require any conductive outer periphery to be employed, but uses a unique contact configuration operative with a conductive area on the inner periphery of an annular or toroidal member.